1. Field of the Invention
The present invention relates to a method of assembling optical components, such as LD modules and in-line isolators, comprising a plurality of metal housings holding optical elements, and relates to such optical components per se.
2. Description of the Related Art
Conventionally, to connect fine optical elements optically, these optical elements are stored and held in metal housings first, and the mutual positional relationship between these metal housings is adjusted by adjusting the optical axes so that an optimum position where decrease of light-intensity is minimized is determined. After that, the metal housings are bonded by spot-welding.
For example, according to Japanese Patent Unexamined Publication No. Hei-7-318763, as shown in FIG. 6, a first metal housing 5 holding an optical fiber 7, which is one optical element, through an optical fiber holder 6, and a second metal housing 1 including an optical element 3, which is the other optical element, are spot-welded by welding spots 10 on the outer circumferences of the joint surfaces where the metal housings abut against each other.
In addition, as understood from FIG. 6, a member which is located in a lower position is made to more project laterally than a member located in an upper position to thereby form a recess portion, so that welding can be formed easily because of the existence of the recess portion and the surface tension of welding metal.
Although the direction of radiation of a laser beam is not disclosed in this publication, conventionally, a laser beam 31 is radiated on the recess portion directly.
However, in the conventional bonding method for making optical components, there is a problem that thermal deformation appears in the metal housings in the process to weld the metal housings after the optimum position between the metal housings is determined by adjusting the optical axes to minimize the decrease of light-intensity, and consequently the position determined by the adjustment of the optical axes before welding may be displaced to thereby decrease light-intensity.
In order to solve the problem of displacement between the optical fiber holder 6 holding the optical fiber 7 and the first metal housing 5, the above-mentioned Japanese Patent Unexamined Publication No. Hei-7-318763 makes a proposal in which, instead of the first metal housing 5, a special spacer is inserted between the optical fiber holder 6 and the second metal housing 1 so as to weld the respective abutting surfaces with each other by spot-welding.
However, the problem of positional displacement between this special spacer and the second metal housing 1 caused by thermal unevenness of the welding portions was not solved even by this proposal.
Therefore, the present inventors investigated the cause of the above-mentioned thermal deformation in spot-welding and the positional displacement due to the thermal deformation. On the basis of the result of this investigation, the present inventors disclose an optical compoenent assembling method and the optical components per se in which decrease of light-intensity when metal housings are welded can be suppressed within an allowable range at low costs, and without requiring any special parts.
An experimental examination was made about the cause of the thermal deformation caused by welding the above-mentioned metal housings by the conventional welding method, and the positional displacement between the metal housings.
First, investigation was made about the thermal deformation in the case where the respective surfaces to be bonded of two cylindrical metal housings 10 and 20, as shown in FIG. 3A, the outer diameters of which are different from each other, were made to abut against each other while their centers were made coincident with each other so that the side of the first metal housing 10 overhung from the side of the second metal housing 20 with its overhanging quantity xcex94X equal all over the circumference, and spot-welding was performed in positions which were on the outer circumferences of the joint surfaces where the metal housings abut against each other, and which were rotationally symmetrical with respect to the center of the joint surfaces.
The welding positions herein used means positions 30a, as shown in FIG. 1, which are on the outer circumferences of the joint surfaces where the respective metal housings 10 and 20 abut against each other, and which are rotationally symmetrical with respect to the center of the joint surfaces, and which are on the bottom of V-shaped recess portions formed by a skirt portion of the overhanging metal housing 10 and a side 20a near the joint surface of the other metal housing 20.
The present inventors have found that, though thermal deformation occurs in the spot-welded metal housings in the direction parallel to the direction (hereinafter referred to as xe2x80x9caxial directionxe2x80x9d) of virtual central axis (hereinafter simply referred to as xe2x80x9ccentral axisxe2x80x9d) perpendicularly crossing the joint surfaces where the metal housings abut against each other at the center position of the joint surfaces so as to form a fine space, enough deformation to tilt the central axes of the spot-welded metal housings does not occur, as shown in FIG. 3A.
This is because the rate of heat supply by radiation of a laser beam and the rate of cooling are equal between the respective welding positions in the left and right circles A and B of FIG. 3A. That is, when a laser beam is radiated onto the welding positions under the same conditions, first, metal at the irradiated positions is heated and melted, and heat is diffused in the inside of the metal housings so that the metal housings expand thermally.
Therefore, the melted portions at the respective welding positions are subjected to tensile stress in the axial direction of the metal housings so as to be extended due to the thermal expansion. Next, because the cooling starts from the outer-circumferential portions of the metal housings including these extended melted portions, the melted portions are solidified while being extended in the axial direction.
In this case, the external surface areas around the welding positions within the circles A and B of FIG. 3A were equal and the cooling rates of the two portions were also equal. Accordingly, it is considered that the expansion and shrinkage of the left welding position were equal to those of the right welding position and that, through deformation parallel to the axial direction was caused, deformation enough to tilt the central axis was not caused. Generally, decrease of light-intensity is so small as may be ignored in the case of deformation in the direction parallel to the central axis, while it is comparatively large in the case of deformation in the direction to tilt the central axis.
In addition, because the inside of the metal housings is cooled more slowly than the outside, cooling advances in the inside even after the welding portions facing the outside are solidified. As a result, a fine space is produced between the bonded surfaces of the metal housings 10 and 20.
Next, because the central axes of the respective metal housings determined by the optical axes adjustment are generally shifted, that is, the overhanging quantity xcex94X of the overhanging metal housing is often not uniform in every position in the outer circumferences of the joint surfaces, investigation was made about thermal deformation in the case where the metal housings were welded in this state.
In this case, it is considered that, as shown in FIG. 3B, thermal deformation in the direction to tilt the central axis is produced by welding because of unevenness in cooling rate among the welding positions when the overhanging quantity xcex94X of the side of one metal housing 10 relative to the side of the other metal housing 20 is different among the positions of the joint surfaces where the metal housings abut against each other.
In the area around the welding position within the circle A in the left of FIG. 3B, the overhanging quantity xcex94X of the metal housing 10 is comparatively small in comparison with the area around the welding position within the circle B in the right of FIG. 3B, so that the ratio of the outside surface area per volume in which metal in the welding position faces the outside is comparatively small. Accordingly, it is difficult for heat to escape and therefore the rising of temperature of the metal housing and the thermal deformation become relatively large.
On the other hand, in the area around the welding position within the circle B in the right of FIG. 3B, the overhanging quantity xcex94X is comparatively large, and the ratio of the outside surface area per volume of metal in the welding position is comparatively large. Accordingly, heat is apt to escape so that the rising of temperature of the metal housing and the thermal deformation become relatively small.
As a result, the inventors found the quantity of thermal deformation in the welding position within the circle A in the left of FIG. 3B becomes larger than the quantity of thermal deformation in the welding position within the circle B in the right. Accordingly, welding causes thermal deformation in the direction to tilt the central axis, that is, positional displacement in the direction to rotate the optical axis to the right.
If the metal housings are pressed by a force against the thermal expansion from above and under so as to entirely suppress the thermal expansion in the axial direction at the time of welding, there is a possibility to exclude such thermal deformation itself. However, buckling is produced in the metal housings to thereby injure optical elements held in the inside of the metal housings. Accordingly, this method cannot be adopted.
Though a large effect cannot be expected, it can be considered that, if the thermal expansion is suppressed by pressing the metal housings within an allowable range, the thermal deformation and the decrease of light-intensity due to the thermal deformation can be suppressed so much.
The present invention has been made to solve the above problems, and therefore an object of the present invention is to provide an optical component assembling method in which even if the central axes of respective metal housings determined by the optical axes adjustment do not coincide with each other, thermal deformation in the rotational direction caused by welding thereafter can be suppressed, so that the decrease of light-intensity is extremely small and provides such optical components.
Because the central axes of the metal housings which are in the mutual optimum positions determined by the optical axes adjustment are often incoincident with each other, the present invention is suitable for the case where optical components are mass-produced and has a great practical significance.
In order to solve the above problems, according to the present invention, provided is an optical component assembling method, in which a first metal housing holding a first optical element and a second metal housing holding a second optical element are made to abut against each other on joint surfaces thereof, and irradiated with a laser beam so as to be fixedly welded with each other, characterized in that a side of one of the metal housings is made to overhang from a side of the other of the metal housings, and the laser beam is radiated in an incident direction inclined to the overhanging metal housing side so as to perform spot-welding on outer circumferences of the joint surfaces where the metal housings abut against each other.
That is, on the basis of the above-mentioned investigation and consideration about thermal deformation and positional displacement, the present inventor obtained a view that the decrease of light-intensity after welding can be suppressed even when the central axes of the respective metal housings adjusted by the optical axes adjustment are shifted perpendicularly to each other, if heating and cooling conditions are made as equal as possible among respective welding positions, and the heating and cooling rates are made moderate. Through trial and error repeated on the basis of this view, the present inventor has completed this invention.